1. Field of the Invention
The invention relates to a digital communications system and more specifically to modulation methods facilitating fading distortion compensation that performs a quasi-synchronous detection using a pilot signal.
2. Description of the Prior Art
In a digital communications system, especially in a digital mobile radio system, the envelope of information signals or symbols is distorted (i.e., the phases and the amplitudes of information signals involves phase errors and amplitude errors) due to fading. The phase error of the received signal causes an error in the frequency of a local oscillator in the receiver. The error of the local oscillation frequency with respect to the carrier frequency is hereinafter referred to as a xe2x80x9cfrequency offsetxe2x80x9d. The phase error (or frequency offset) and the amplitude error of the received signal have to be estimated and compensated for in the receiver.
A fading distortion compensation scheme using a pilot signal is described by S. Sampei, xe2x80x9cRayleigh Fading Compensation Method for 16-QAM MODEM in Digital Land Mobile Radio Systems,xe2x80x9d Trans. IEICE (The Institute of Electronics, Information and Communication Engineers) Japan, Vol. J72-B-II, No. 1, January 1989, pp. 7-15 (which is hereby incorporated by reference). FIG. 1 is a diagram showing a signal constellation used in this 16-QAM system. In FIG. 1, small black-filled circles indicate 16 signal points in an in-phase (I) and quadrature-phase (Q) plane. One of the signal points with the maximum amplitude, that is, any of the signal points A, B, C and D is assigned to a pilot signal. (Since one of the 16 signal points in the signal constellation is used for a pilot signal, the remaining 15 points are available for the information signals.) A pilot signal is inserted in every frame or every Nxe2x88x921 information symbols (assuming that N symbols constitute one frame) in a transmitter. The estimation and compensation of distortions (due to fading) of information signals or symbols are achieved by interpolation using the pilot signals.
In such quasi-synchronous detection as just described, larger-amplitude pilot symbols yields a higher precision in estimation of the frequency offset and the amplitude error of the information signals, which results in an improvement of the bit error rate, which is a function of the ratio of the carrier signal power to the noise power density per one symbol. However, enlarging the amplitude of the pilot signals without taking any measure lowers the power efficiency of the power amplifier of the transmitter system due to an increase in the ratio of the peak to the average transmission power.
It is therefore an object of the invention to provide a method of and a system for compensating for fading distortion of the received signal with a raised precision and thereby to provide a digital communication system that permits a reception of a reduced bit error rate.
In accordance with the principles of the invention, a point that differs in phase from any of the signal points for possible information symbols and that is larger in amplitude than any of the signal points is selected for a pilot signal point in a signal constellation (or a signal point map plotted on a in-phase and quadrature-phase plane). A pilot signal is inserted in every frame or every predetermined number of information signals.
In a receiver, the fading distortion of each of the pilot signals regularly inserted in the received signals is determined. The fading distortions of the information signals are estimated by interpolation using the determined fading distortion of the pilot signal, and then compensated for.
In a preferred embodiment, the amplitude of the pilot signal is set not larger than 1.6 times a maximum possible amplitude of the information signals.
The frequency band of each information signal is preferably limited with a roll-off filter with a roll-off coefficient ranging from 0.1 to 0.4.